A liquid crystal display device has been widely used in a TV, a monitor for personal computers, and the like, because it is a display device with low power consumption and it can be reduced in weight and thickness. However, according to the liquid crystal display device, light polarization is generally controlled by a tilt angle of a liquid molecule in accordance with an applied voltage, and therefore the light transmittance depends on a viewing angle. Therefore, in the liquid crystal display device, a contrast ratio is reduced and gradation reversal at the time of intermediate scale display, and the like, are caused, depending on the viewing angle. Accordingly, such a common liquid crystal display device has room for improvement in that the viewing angle characteristics are insufficient.
An alignment division technique in which alignment and tilt directions of liquid crystal molecules are divided into two or more regions in one pixel has been developed. According to this technique, if a voltage is applied to a liquid crystal layer, the liquid crystal molecules are tilted in different directions in the pixel, thereby improving the viewing angle characteristics of the liquid crystal display device. The respective regions which differ in the alignment azimuth of the liquid crystal molecules are each referred to as a domain. The alignment division is also referred to as multi-domain.
With regard to the liquid crystal mode where the alignment division is performed, examples of horizontal alignment mode include multi-domain twist nematic (TN) mode, multi-domain electrically controlled birefringence (ECB) mode, and multi-domain optically compensated birefringence (OCB) mode. In addition, multi-domain vertical alignment (MVA) mode, patterned vertical alignment (PVA) mode, and the like are mentioned as a vertical alignment mode. Various modifications have been made to further improve the viewing angle in the liquid crystal display devices in various modes.
A rubbing method, a photo alignment method, and the like, may be mentioned as such an alignment division method. With regard to the rubbing method, an alignment division method of separating a rubbing region from a non-rubbing region by patterning a resist has been proposed. However, according to such a rubbing method, an alignment film surface is provided with an alignment treatment by being rubbed with a cloth wound on a roller, which causes the following defects: damage on switching elements by a fiber of the cloth, dusts such as rubbed scraps, or static electricity; characteristic shift; and characteristic deterioration. In such a point, the rubbing method still has room for improvement.
In contrast, the photo alignment method is an alignment method in which a photo alignment film is used as a material for the alignment film, and the photo alignment film is irradiated with a light beam such as UV, thereby being provided with an alignment regulating force. Accordingly, the alignment film can be provided with the alignment treatment in a contactless manner. Therefore, generation of soils, dusts, and the like during the alignment treatment can be suppressed. In addition, use of a photomask at the time of exposure makes it possible to irradiate the alignment film with a light beam under conditions which vary depending on a region in the alignment film surface. As a result, domains having a desired design can be easily formed.
As a conventional alignment division method using the photo alignment method, the following method may be mentioned if one pixel is divided into two domains. A method in which a half region of the pixel is subjected with the first exposure using a photomask including a transmissive part and a shielding part corresponding to each pixel, and then the photomask is shifted by about a half-pitch, and then the rest region of the pixel is subjected to the second exposure under conditions different from those in the first exposure. According to such a photo alignment method, each pixel can be easily divided into two or more domains using the photomask. For example, the Patent Document 1 discloses a technology of VAECB (vertical alignment ECB) mode in which an alignment treatment is performed by the photo alignment method.
In addition, an increase in size, particularly in the liquid crystal display device, has rapidly proceeded recently. Liquid crystal TVs in 40 to 60-inch model have rapidly developed, although plasma TVs conventionally accounted for the greatest share of the devices in such a size. However, it is very difficult to perform the alignment division in such a 60-inch liquid crystal display device by the above-mentioned conventional photo alignment method. The reason is given below. An exposure device which can be installed in a factory is limited, and therefore it is realistically impossible to install an exposure device capable of completing exposure for the 60-inch substrate by one exposure. That is why it is impossible to complete exposure for the entire surface of the 60-inch substrate by one exposure. Accordingly, the exposure for the substrate needs to be completed through several exposures, when a large liquid crystal display device is subjected to the alignment division. Further, also if the alignment division treatment is performed for a relatively small liquid crystal display device in 20-inch model by the photo alignment method, the exposure for the substrate might need to be completed through several exposures in the case where the size of the exposure device needs to be decreased as much as possible. However, if the exposure for the substrate is completed through several exposures and thereby a liquid crystal display device is prepared, a joint line between the exposure regions is clearly observed on the display screen.
Accordingly, if the liquid crystal display device is subjected to the alignment division by completing the exposure for the substrate through several exposures, there is still room for improvement in that generation of the joint line on the display screen is suppressed and the yield is improved.
[Patent Document 1]
Japanese Kokai Publication No. 2001-281669